This invention relates to porous, expanded PTFE (polytetra-fluoroethylene) and, in particular, to an improved process for imprinting a predetermined texture on porous, expanded PTFE.
Experiments in the early 1900's established venous and arterial autografting (replacing a section of a patient's blood vessel with a section of vein from elsewhere in the patient) as an effective technique for replacement of damaged or defective blood vessels. However, the need went beyond what could be treated by this technique, leading to a search for artificial or prosthetic veins and arteries for implanting in the vascular system. The need includes not only replacements for veins and arteries but also grafted blood vessels which can withstand repeated puncturing, e.g. for patients undergoing hemodialysis.
At present, synthetic vascular grafts are made by knitting or weaving synthetic materials, such as Dacron.RTM., or by stretching or expanding PTFE (Teflon.RTM.). The different precesses for making grafts produce grafts of different porosity; i.e. woven Dacron grafts are the most porous and expanded PTFE is the least porous. Porosity affects how easily the graft is sutured, how much leakage there will be during and after surgery, and how well or quickly tissue can incorporate the graft after surgery.
Because of the other desirable properties of PTFE, e.g. stability and compatibility, it has long been desired to increase the porosity of expanded PTFE. Expanded PTFE is produced in either tubular or sheet form for use as vascular grafts. In tubular form, PTFE is used to replace defective or damaged veins or arteries. In sheet form, sections of the sheet are cut to size and sutured to a vein or artery as a patch to repair or close an incision in the wall of the vessel. A more porous form of expanded PTFE would have many additional uses, e.g. for filtering particles from liquid or gas, for gas separation, for controlled or damped release of gas or liquid, for wrapping or bandaging.
As disclosed for example in U.S. Pat. No. 4,187,390--Gore (herein the "Gore" patent), expanded PTFE is typically made by a cold extrusion process in which a paste of PTFE and lubricant is forced under pressure through a shaped nozzle. The extruded sheet is calendered by pressing the sheet between smooth rollers. The sheet thins and spreads and may be passed through several sets of rollers until it achieves the desired thickness, e.g. 0.4-0.6 mm. Since calendering increases the tensile strength of the sheet in the direction of movement through the rollers, the sheet is usually rotated between passes to increase tensile strength in all directions. The sheet is then expanded and kept expanded during "sintering," i.e. raising the temperature of the sheet approximately to its melting point, approximately 340.degree. C., and then allowing the sheet to cool. After sintering, the sheet retains its shape and is ready for use. It is desired that any technique for increasing the porosity of PTFE be compatible with this process.
Expanded PTFE has a microscopic structure of nodes interconnected by fibrils and is normally not very porous. One measure of porosity is dimensional, e.g. 8-10 microns. Unlike most other polymers, for PTFE this dimension is not the diameter of a hole or pore through the sheet but is the distance from one node to another among a plurality of nodes making up a pore. Since the nodes are interconnected by fibers, the dimension is a measure of fiber length.
On a macroscopic level, a patch of such material feels less pliable, e.g. wraps less easily, and is more difficult to puncture than other graft materials.
In view of the foregoing, it is therefore an object of the invention to provide a vascular graft made from expanded PTFE having a higher porosity than has been obtainable in the prior art.
A further object of the invention is to provide an expanded PTFE sheet having both high porosity and high tensile strength.
Another object of the invention is to provide an expanded PTFE sheet in which porosity is more uniform.
A further object of the invention is to provide an expanded PTFE sheet having high porosity and high suture strength.